Intramolecular Hetero and Diels-Alder Reactions

The intramolecular Diels-Alder (IMDA) or hetero Diels-Alder (IMHDA) cydoad-dition reactions are often the key step in the synthesis of functionalized bicyclic bridged and polycyclic systems with high regio and stereoselectivity. The application of microwave irradiation to IMDA and IMHDA reactions may have significant advantages compared with conventional heating methods. Early examples of microwave activation in these reactions for synthesis of polycyclic frameworks, have been extensively reviewed by de la Hoz and will not be discussed herein [3jj. 

In many of the more recent reports of IMDA and IMHDA cydoadditions, the reactions are performed at high temperature under microwave activation. Different conditions have been used, including closed-vessel processes [56-60] with highly polar solvents or ionic liquids as doping agents in nonpolar solvents, or indeed, microwave irradiation under open-vessel reflux conditions [61-63, 36). 

The IMHDA reaction can be performed efficiently by employing ionic liquids either as solvents or as additives in conjunction vith nonpolar solvents under micro- vave irradiation conditions. Ionic liquids can provide an ideal medium for IMHDA reactions, vhich involve reactive ionic intermediates. Because of the stabilization of ionic or polar reaction intermediates, ionic liquids can afford enhanced selectivities and reaction rates [3k, 64]. 

A (reflux). Toluene 7 days, no conversion, 0% yield MW(210°C), Toluene 500 min, 56% yield 

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